Breast cancer drugs 'impaired by street lights’

Researchers find that lights could make certain cancers resistant to the drug
tamoxifen

Street lights shining through curtains at night could stop breast cancer drugs working properly, research suggests.

The study showed that dim light exposure can make certain cancers resistant to the drug tamoxifen.

However, researchers found that in tests on rats, a melatonin hormone supplement during the night could overcome this.

Prof Steven Hill, of Tulane University School of Medicine, in New Orleans, said: “Resistance to tamoxifen is a growing problem among patients with hormone receptor-positive breast cancer.

“Our data, although they were generated in rats, have potential implications for the large number of patients with breast cancer who are being treated with tamoxifen, because they suggest that night-time exposure to light, even dim light, could cause their tumours to become resistant to the drug by suppressing melatonin production.”

The study did not identify how much light exposure it would take to suppress this production. But Prof Hill said: “We think it could be as little as the amount of light that comes in the bedroom window from a street light.”

He cautioned against patients supplementing naturally produced melatonin. “Melatonin is produced by our bodies exclusively during darkness at night, and taking melatonin supplements at the wrong time of day would potentially disrupt the circadian [sleep-wake cycle] system, particularly the natural melatonin cycle, which may, in itself, impair breast cancer responsiveness to tamoxifen.”

The research, published in the journal of the American Association for Cancer Research, looked at rats living in 12 hours of light followed by 12 hours of darkness, or simply 12 hours of normal light followed by 12 of dim light.

Prof Hill said: “Our levels of melatonin are not determined by sleep, as many people think.

“It is actually the darkness that is important. During the night, if you sleep in a brightly lit room, your melatonin levels may be inhibited; however, if you are in the dark but cannot sleep, your melatonin levels will rise normally.”